1. Field of the Invention
The present invention relates to a laminated body of cellulose fine fiber which can be used as a coating agent or a functional layered material etc. The invention also relates to a manufacturing method of such a laminated body.
2. Description of the Related Art
In recent years as concerns about environmental problems grow, various kinds of natural polysaccharide such as naturally-derived starch, cellulose, chitin and chitosan etc. and their derivatives have attracted attention as a biomass. In addition, basic materials made of a biodegradable resin, which is environmentally degradable to water and carbon dioxide, also attracts attention, and are commercially supplied. Specific examples are microbial aliphatic polyesters, various kinds of natural polysaccharide such as naturally-derived starch, cellulose, chitin and chitosan etc. and their derivatives, biodegradable resins fully-synthesized by a chemical method, and a PLA (polylactate) obtained by polymerizing lactic acid made from starch etc.
Among these, cellulose, which is produced most on the earth, attracts attention because of fibrous form, high crystallinity, a high level of strength, a low linear expansion coefficient, chemical stability and excellent safeness for a living body. In particular, fine cellulose fibers have been eagerly developed in recent years at the prospect of being used as various kinds of functional materials such as a wrapping (or packaging) material etc.
Regarding manufacturing method of a fine cellulose fiber, for example, a method of obtaining the fine cellulose fiber by oxidizing some of the hydroxyl groups in cellulose with TEMPO (2,2,6,6-tetramethylpiperidinyloxy radical) as a catalyst followed by dispersing the resultant oxidized cellulose in a medium is disclosed in the Patent document 1 below. This method makes it possible with relative ease to obtain a fine cellulose fiber having a crystal structure of cellulose I type by utilizing electric repulsions between negatively-charged carboxy groups.
In addition, the Patent document 2 discloses a method of dispersing the oxidized cellulose by the TEMPO oxidation to prepare a gas barrier material containing fine cellulose fiber with an average diameter of 200 nm or less, followed by coating the gas barrier material on a substrate such as PET film and PLA etc. and drying the resultant material so as to obtain a formed-composite having gas impermeability.
Patent document 1: JP-A-2008-1728.
Patent document 2: JP-A-2009-57552.
However, films made from an aqueous dispersion liquid of fine cellulosed fiber as is described above have problems of low adhesion to the substrate due to their low reactivity and a small area of contact to the substrate, which is caused by a rigid nature, high elastic modulus and fibrous shape of the fine cellulose fiber. In addition, there are also problems in terms of repellency, wettability, inhomogeneity and coatability since polar groups such as carboxy group etc. are often introduced to the fine cellulose fiber and an aqueous solvent is usually used. For example, the low adhesion to the substrate causes delamination when the films are formed on the substrate as a layer of a laminate. In addition, the problems of repellency, wettability and inhomogeneity and low coatability further causes a problem of difficulty in obtaining a continuous and homogenous film, a problem of low printability and processing suitability, a problem of poor optical properties, and a problem of difficulty in obtaining good performance when the formed film is used as a barrier material. Moreover, in the case where paper or PLA etc. is used as the substrate, the substrate and the formed film may degrade as time passes and their adhesion may become worse since paper and PLA etc. are a natural product, which are known to have less wettability to the aqueous solvent and less adhesion to the formed film than petroleum-derived synthetic-resins. Accordingly, it has been difficult to evenly coat an aqueous dispersion liquid of fine cellulose fiber on a substrate, and further, to achieve adhesion to the substrate. Moreover, it has been also difficult to ensure adhesiveness and to prevent degradation of the substrate and the fine cellulose fiber even after time passes.